In the past few years rising costs have prodded building systems engineers to devise more efficient systems to control heating and cooling in office buildings. One approach, well known in the art, employs a single air handling system common to several tenants from which conditioned air can be obtained as needed. In such a system a method must be devised that accurately apportions cost among tenants according to actual consumption. Consequently, the control system must detect the movement of air in each plenum.
A conventional approach involves an airflow detection scheme with sensors placed locally to determine when airflow occurs in an individual plenum. The traditional sensors employed have been transducers which detect airflow and convert mechanical motion into an electrical signal.
The choice of mechanical to electrical transducers places constraints on any overall control device. A significant amount of electrical power is required to keep each sensor operational, with the required power increasing with increasing numbers of sensors. These sensors are susceptible to standard environmental constraints including the effects of temperature, moisture, and breakdowns in electrical insulation. In addition, conventional copper wire occupies a significant amount of space in building service cores.
A most significant drawback to a system using these sensors is that each sensor must be electrically wired in parallel. Although serial wiring of sensors requires less than half of the cable needed for parallel wiring, an array of traditional mechanical to electrical transducers could be rendered useless if one should fail. Consequently, a parallel sensor array with at least two wires per sensor is mandated. Logistically this arrangement becomes quite cumbersome with large numbers of sensors.
Another detection apparatus employs an optical fiber as the means for conveying sensor information. Optical fibers have the advantages that they are light, less bulky, require less space, and are less subject to environmental intererence than their copper wire counterparts. However, schemes currently outlined for transducing airflow information onto a propagating light signal requires an elaborate or costly transducer. Indirect transducers would entail a mechanical to electrical transducer in conjunction with one that transduces the electrical signal to an optical one. In addition, such a transducer still requires parallel wiring to avoid the inherent potential system failure that accompanies serial sensor configurations.
Direct mechanical to optical transducers such as one using a microbending technique as disclosed in U.S. Pat. No. 4,463,254 are not amenable to detect sensitive airflow movement without elaborate modification.